Patent Application
20090219102
Electrical oscillators are used in digital systems, communications systems and electronic test equipment, to name only a few applications. One type of electrical oscillator is known as a voltage controlled oscillator (VCO). A VCO is a component that can be used to translate DC voltage into a time dependent voltage or signal. In general, VCOs are tunable oscillators designed to produce an oscillating signal of a particular frequency ‘f’ corresponding to a given tuning voltage. The frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage applied to the oscillator. The frequency ‘f’ may be varied from fmin to fmax and these limits are referred as the tuning range or bandwidth of the VCO. For many applications, particularly for test instrumentation and communication systems, a comparably wide tuning range is beneficial.
Many VCOs incorporate varactors, which are reverse biased diodes that function as voltage controlled capacitors, as the tuning mechanism. Varactors are comparably small, low cost, use negligible bias power and are available as integration elements in some semiconductor processes. Varactors are used in conjunction with fixed inductors to realize tunable LC resonators. The quality (Q) factor (or simply, Q) of varactors is usually high at low frequencies and degrades as with increasing frequency. While a tuning bandwidth of more than an octave is common in varactor-based VCOs at low frequencies, at microwave frequencies and above (i.e., frequencies greater than about 10 GHz) it is difficult to achieve a tuning bandwidth of more than one octave. Thus, the tuning range can be undesirably limiting.
As is known, varactor-tuned VCOs have modest phase noise at microwave frequencies. When lower phase noise is required, tunable high-Q Yttrium-Iron-Garnet (YIG) resonators are often used. Alternatively, when low phase noise is not a requirement, multivibrator VCOs can be used. Multivibrators do not include tunable resonators, but rely on varying the current charging a fixed capacitor to tune the oscillation frequency. There are two main advantages in using a multibrator. First, multivibrators do not require varactors, which simplifies the circuit and makes multivibrators suitable for integration in semiconductor processes that do not have varactors. Second, they have very wide tuning range, typically multi-octave.
While multivibrator-based VCOs have a greater tuning range than varactor-based VCOs, their tuning range can nonetheless be limited at high frequencies due to non-ideal behavior of the active devices. For example, transistors become less unilateral and their gain decreases with frequency (due to parasitic transistor elements), preventing multivibrators from achieving a wide tuning range.
Another disadvantage of many known VCOs is that the oscillation amplitude is typically established by the limiting action of the non-linear active device characteristics, which for some bipolar transistors can cause the transistor to operate in an unreliable saturation mode.
There is a need, therefore, for electrical oscillators, including VCOs that overcome at least the shortcoming of known oscillators described above.
In accordance with a representative embodiment, an electrical oscillator includes a first oscillating transistor and a second oscillating transistor. The electrical oscillator also includes a first non-linear load connected to a terminal of the first oscillating transistor and a second non-linear load connected to a terminal of the second oscillating transistor. The electrical oscillator also includes a negative resistance generated between the terminal of the first oscillating transistor and the terminal of the second oscillating transistor, wherein the electrical oscillator does not include a tunable resonator.
In accordance with another representative embodiment, a voltage controlled oscillator (VCO) includes a first oscillating transistor and a second oscillating transistor. The VCO also includes a first non-linear load connected to a terminal of the first oscillating transistor and a second non-linear load connected to a terminal of the second oscillating transistor. The VCO also includes a negative resistance generated between the terminal of the first oscillating transistor and the terminal of the second oscillating transistor, wherein the VCO does not include a tunable resonator.
The present teachings are best understood from the following detailed description when read with the accompanying drawing figures. The features are not necessarily drawn to scale. Wherever practical, like reference numerals refer to like features.
It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting.
As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices.
As used in the specification and appended claims, the term tunable resonator includes a resonator tuned thermally, mechanically, electrically or magnetically. Examples of such tunable resonators include, but are not limited to: varactors, YIG resonators, cavity-tuned resonators and dielectric resonant oscillators (DROs).
In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. Descriptions of known systems, devices, materials, methods of operation and methods of manufacture may be omitted so as to avoid obscuring the description of the example embodiments. Nonetheless, systems, devices, materials and methods that are within the purview of one of ordinary skill in the art may be used in accordance with the representative embodiments.
As described more fully herein, the representative embodiments relate generally to electrical oscillators having oscillating transistors, with a terminal loads comprising a negative resistance. As will become clearer as the present description continues, the negative resistance may be a differential negative resistance. Frequency tuning of the oscillating transistors comprises varying an impedance of a positive feedback connection. While the embodiments are described primarily in connection with VCOs, electrical oscillators in general are contemplated. For example, rather than varying the impedance of a positive feedback connection via an applied voltage or current, the electrical oscillators of representative embodiments may be tuned by varying the impedance of the terminal (e.g., collector) loads by varying the temperature of the load or by varying the intensity of light shining on the load.
The first oscillating transistor 101 includes a first collector 112 and the second oscillating transistor 102 includes a second collector 113. The first collector 112 is connected to a first tap 114, which in turn is connected to an emitter of a second diode-connected transistor 116. The second collector 113 is connected to a second tap 115, which in turn is connected to an emitter of a first diode connected transistor 117. The first diode-connected transistor 117 and the second diode-connected transistor 116 may alternatively be diodes, and are often referred to below as first diode 117 and second diode 116 for ease of description. As will become clearer as the present description continues, the diodes 116, 117 are substantially identical in performance.
The first diode 117 includes a first collector resistance 118 and a second collector resistance 119 connected differentially as shown. Likewise, the second diode 116 includes a third collector resistance 120 and a fourth collector resistance 121 also connected differentially. Completing the circuit is an input 122 for the collector voltage, which is at ground in the present configuration; and positive output terminal 123 and negative output terminal 124.
The first and second oscillating transistors 101, 102 are illustratively npn InP heterojunction bipolar transistors (HBTs). However, this is merely illustrative, and it is emphasized that other three terminal devices are contemplated by the present teachings to provide oscillation. For instance, HBTs based on other materials (e.g., other III-V semiconductors) may be used. Alternatively, pseudomorphic high electron mobility transistors (pHEMTs) may be used for the first and second oscillating transistors 101, 102. Alternatively, field-effect transistors (FETs) may be used for the first and second oscillating transistors 101, 102. Illustrative FETs include metal-oxide-semiconductor (MOS) FETs may be used. Moreover, metal semiconductor FETs (MESFETs) may be used. Again, a wide variety of materials are available for fabricating the transistors 101, 102, including but not limited to Si, Ge, SiGe, and a variety of III-V semiconductors.
Similarly, as noted above the first and second diodes 116, 117 may be diode-connected transistors or diodes. If diode-connected transistors, the first and second diodes 116, 117 may one of the types of transistors described above with the shorting of two terminals to effect the diode. Alternatively, one of a variety of pn-junction diodes or metal-semiconductor junction (Schottky) diodes may be used for the first and second diodes 116, 117.
Selection of alternative devices (e.g., FETs, pnp transistors) may require modification of parameters, connection, etc. to realize a functioning oscillator. For instance, if a FET is selected, rather than a collector, the drain of the first and second oscillating transistors 101, 102 would be connected to a negative resistance. As such, more generally therefore, a terminal of a three-terminal device is connected to the negative resistance. As one deft in circuit design will appreciate the need for such modifications, these modifications are thus contemplated by the present teachings.
Finally, and as will be appreciated by one of ordinary skill in the art, the fabrication of the circuit 100 in large-scale processing is advantageous. Thus, in certain embodiments employing wafer-scale fabrication, the selection of materials is predicated on the selection of devices for the first and second oscillating transistors 101, 102 is related to the selection of the diodes 116, 117. As such, if one were to select a GaAs-based HBT for transistors 101, 102, the diodes likely would be GaAs-based diodes as well.
In operation, the cross-connection of the first collector 112 to the second diode 116 and the second collector 113 to the first diode 117 as shown results in a differential load at each collector, and a negative resistance. The differential loads of the circuit 100 comprise non-linear terminal (collectors in the presently described embodiments) loads comprising of resistors 118-121 and first and second diodes 117, 116. The connection of the second diode 116 via the tap 114 provides a bias voltage from the first transistor 101 to the second diode 116; and the connection of the first diode 117 via the tap 115 provides a bias voltage from the second transistor 102 to the first diode 117.
In the representative embodiments described in conjunction with
The non-linear terminal load (e.g., non-linear collector load) of the representative embodiments function as a limiting mechanism for the oscillator. To this end, at lower oscillation amplitudes the closed loop gain of the oscillator 100 is greater than unity (1), which allows the oscillation to start. The limiting action provided by the non-linear terminal loads reduces the gain to unity and the amplitude of oscillation stabilizes at the final oscillation condition. Thus, the non-linear collector loads function as stabilizing limiters since the closed-loop gain decreases as the oscillation amplitude increases.
Decreasing the gain reduces the tendency of the first and second oscillating transistors to oscillate and results in a substantially stable oscillation amplitude. In accordance with the presently described embodiments, the limiting action is manifest as a decrease in resistance across the first and second diode-connected transistors 117, 116 (or alternatively diodes) as the oscillation amplitude increases. As will be appreciated by one of ordinary skill in the art, the limiting action is a function of the voltage across the first and second diode-connected transistors 117, 116; and a function of the DC bias through the first and second diode-connected transistors 117, 116.
In the presently described embodiments, the voltage across the first and second diodes 117, 116 is illustratively an RF voltage. Increasing the forward DC bias current through the first and second diodes 117, 116 limits oscillation to a lower RF voltage level. Moreover, lowering the load impedance increases the frequency of oscillation (as can be seen by the negative resistance curves in
Certain clear benefits are provided by the electrical oscillator 100. In a typical oscillator the limiting action is provided by the oscillating transistors. By contrast, in accordance with the representative embodiments, by having a limiting action that does not rely on or otherwise comprise additional or external oscillating transistors, the first and second oscillating transistors 101, 102 can be biased for optimum high frequency operation, or maximum signal-to-noise ratio, or both, without regard to the desired limiting RF amplitude level. Moreover, the first and second oscillating transistors 101, 102 are able to operate in a substantially linear mode which can improve reliability and maintain the loaded Q of the oscillator.
In illustrative embodiments, varying the differential impedance of the cross-connected first and second oscillating transistors 101, 102 and thereby tuning the electrical oscillator 100, involves adjusting the tuning voltage at the input 105. Specifically, as the tuning voltage, Vtune, is made more negative, the emitter current increases in the first and second oscillating transistors 101, 102 and the voltage across the second collector resistance 121 and the fourth collector resistance 121 also increases. This increases the forward bias across first and second diodes 117, 116, respectively. As such, as Vtune is made more negative, the differential impedance between the collectors of the first and second oscillating transistors 101, 102 decreases due to decreased load impedance and increased capacitance.
As described above, the tuning of the electrical oscillator 100 is effected by varying the differential impedance presented to the collectors of the cross-connected first and second oscillating transistors 101, 102. In another representative embodiment, an external bias voltage is applied to increase the tuning range of the oscillator.
The input 201 provides a bias voltage, Vbias
As will be appreciated, in the embodiments described in connection with
In accordance with another representative embodiment, a photoresistor (not shown) or similar light-dependent resistor (not shown) could supplant the diodes 116, 117. Variation of the intensity of light directed to the photoresistor will result in a variation of the impedance at the terminals (e.g., collectors 112, 113). Like the thermistors, the photoresistors provide the limiting function that reduces the impedance with increasing power. Still other devices and configurations for frequency tuning of the oscillating transistors 101, 102 by varying an impedance of a positive feedback connection within the purview of one of ordinary skill in the art are contemplated.
However, the negative resistance curve 301 is a function of collector voltage and will ‘move’ toward the ordinate (in an ‘upward’ direction 303) towards the top of the polar plot as the amplitude of oscillation increases. The model shown in
The negative resistance curves 301, 401 are functions of oscillation frequency and the oscillation frequency depends on the value of the collector load impedance
In view of this disclosure it is noted that variations to the electrical oscillators and VCOs described herein can be implemented in keeping with the present teachings. Further, the various topologies, devices, components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.
